Syrian opposition activists say the bodies of at least 65 people apparently shot in the head have been found in a river in the northern city of Aleppo.
The Britain-based Syrian Observatory for Human Rights posted videos on the Internet on Tuesday, showing dozens of muddied bodies that it says were dragged out of the Queweik river in Aleppo's Bustan al-Qasr district.
Most of the dead appear to be young men in civilian clothes, with gunshot wounds to the head and their hands tied.
The Observatory said it was not clear who carried out the killings. Syrian government and rebel forces both have been accused by rights groups of committing atrocities in the country's civil war.
Activists also said Free Syrian Army rebels and Islamist militants captured a government security compound in the northeastern town of Deir el-Zour on Tuesday, freeing several prisoners after days of heavy fighting.
They said the rebel advance in the oil-rich Syrian region triggered retaliatory air strikes by the government.
There was no government confirmation of the rebel claim.
The U.N. refugee agency said Tuesday the number of Syrian refugees who have fled the conflict into neighboring countries has risen to 700,000.
Speaking in Geneva, UNHCR spokeswoman Sybella Wilkes said the figure includes Syrians already registered as refugees and those awaiting processing in Jordan, Lebanon, Turkey and Iraq. She said aid workers are struggling to cope with the exodus and trying to clear the backlog.
Jordan has seen the biggest influx of refugees, with tens of thousands arriving in the past month.
International donors made pledges of more humanitarian assistance for Syria on Tuesday, a day before joining a U.N. aid conference in Kuwait.
U.S. President Barack Obama said Washington will provide $155 million to help Syrians make it through the winter. He said the aid will include warm clothes for children, medicine for the elderly, flour and wheat for families and blankets, boots and stoves for people huddling in damaged buildings.
The new pledge raises the total U.S. aid commitment to $365 million, making Washington the largest donor of humanitarian supplies to Syria.
"The relief we send doesn't say 'Made in America,' but make no mistake - our aid reflects the commitment of the American people," Obama said. "American aid means food and clean water for millions of Syrians. American aid means medicine and treatment for hundreds of thousands of patients in Damascus, Daraa and Homs. It means immunizations for one million Syrian children. American aid means winter supplies for more than half a million people in Aleppo, Homs and Deir el-Zour. And we are working with allies and partners so that this aid reaches those in need."
EU humanitarian commissioner Kristalina Georgieva said the 27-nation bloc will double its pledges for Syria to about $270 million. She said the aid is "desperately needed" by Syrian people who are "cold, hungry and scared."
In another pledge, an international group of 77 Islamic charities said it will provide $182 million of aid to Syrians affected by the conflict. The non-governmental International Islamic Charitable Organization (IICO) made the announcement after meeting in Kuwait on Tuesday.
U.N. Secretary-General Ban Ki-moon discussed Syria's humanitarian crisis with his Arab League counterpart Nabil Elaraby ahead of the donor conference. A U.N. statement said both men called for an "immediate end to the bloodshed and suffering" of the Syrian people and expressed concern about what they called a "dangerous spill-over" of the conflict in neighboring states.
Deaths in Syria from ongoing conflict